Soil erosion becomes a problem when human activity causes it to occur much faster than under natural circumstances.
Annual soil loss in South Africa is estimated at 300 - 400 million tonnes, nearly three tonnes for each hectare of land. For every tonne of maize, wheat, sugar or other agricultural crop produced, South Africa loses an average of 20 tonnes of soil.
The FAO (Food and Agriculture Organisation, a branch of United Nations) estimates that the global loss of productive land through erosion is 5-7 million ha/year.
Wind and water are the main agents of soil erosion. The amount of soil they can carry away is influenced by two related factors:
(1) Speed - the faster either moves, the more soil it can erode;
(2) plant cover - plants protect the soil and in their absence wind and water can do much more damage.
Plants provide protective cover on the land and prevent soil erosion for the following reasons:
* plants slow down water as it flows over the land(runoff) and this allows much of the rain to soak into the ground;
* plant roots hold the soil in position and prevent it from being washed away;
* plants break the impact of a raindrop before it hits the soil, thus reducing its ability to erode;
* plants in wetlands and on the banks of rivers are of particular importance as they slow down the flow of the water and their roots bind the soil, thus preventing erosion.
The loss of protective vegetation through deforestation, over-grazing, ploughing, and fire makes soil vulnerable to being swept away by wind and water.
In addition, over-cultivation and compaction cause the soil to lose its structure and cohesion and it becomes more easily eroded.
Erosion will remove the top-soil first, once this nutrient-rich layer of soil is gone, few plants will grow in the soil again. Without soil and plants the land becomes desert-like and unable to support life - this process is called desertification. It is very difficult and often impossible to restore desertified land.
To understand soil erosion we must be aware of the political and economic factors affecting land users. In South Africa apartheid policies ensured that 42% of the people lived on 13 % of the land (the"homelands"). This overcrowding has resulted in severe erosion. As the land became increasingly degraded and thus less productive, subsistence farmers were forced to further overuse the land.
The intensive agriculture and overgrazing that followed caused greater degradation. Soil erosion can be seen as both a symptom of underdevelopment (i.e. poverty, inequality and exploitation), and as a cause of underdevelopment.
A reduced ability to produce, invest one's profit and increase productivity, contributes to increasing poverty, and can lead to desertification, drought, floods, and famine.
On commercial farm lands, overstocking, mono-cropping, and the ploughing of marginal lands unsuitable for cultivation has led to soil erosion and desertification. Frequently these practices have been unwittingly encouraged by the state offering subsidies which made it profitable to exploit the land in the short-term.
PREVENTING SOIL EROSION
Preventing soil erosion requires political, economic and technical changes. Political and economic changes need to address the distribution of land as well as the possibility of incentives to encourage farmers to manage their land sustainably.
Aspects of technical changes include:* the use of contour ploughing and wind breaks;
* leaving unploughed grass strips between ploughed land;
* making sure that there are always plants growing on the soil, and that the soil is rich in humus (decaying plant and animal remains). This organic matter is the "glue" that binds the soil particles together and plays an important part in preventing erosion;
* avoiding overgrazing and the over-use of crop lands;
* allowing indigenous plants to grow along the river banks instead of ploughing and planting crops right up to the water's edge;
* encouraging biological diversity by planting several different types of plants together;
* conservation of wetland
In addition to the guidelines above, try the following:
* Pathways can be easily eroded when water flows over them. Prevent this by breaking the water flow with logs, stone packs or old tyres.
* Become a `Erosion Doctor' and repair erosion gullies.
